The present invention relates to the fabrication of semiconductor devices and more particularly to a method for cleaning a semiconductor substrate prior to the formation of a chemically amplified photoresist layer thereon.
The importance of minimizing contamination during semiconductor fabrication processes has been recognized since the early days of the industry. As semiconductor devices have become smaller and more complex, cleanliness requirements have become increasingly stringent, especially for devices with submicron critical dimensions. Cleanliness is especially important during the application of photoresist layers, because contaminants may lead to defects in the photoresist (xe2x80x9cresistxe2x80x9d) profile that then result in slopy etching and poor line definition. In particular, if the profile exhibits defects at its base that result in an abnormal (other than 90 degree) intersection between the resist and the substrate, an etch performed with the defective resist will not produce straight-walled structures, and may result in inoperability of the semiconductor structure.
Poor resist profiles are especially prevalent in chemically amplified resists (xe2x80x9cCA resistsxe2x80x9d) used for microlithography. Traditional resists based on pheno-formaldehyde polymers (novolac resins) do not work well with DUV and laser lithography because they cannot be tuned to respond to the short wavelengths of DUV and laser light (xcx9c300 nm). Chemically amplified resists contain photoacid generators (PAGs) to increase the photosensitivity of the resist. Development of CA resists depends on the catalytic action of the PAG, and if acid loss (neutralization) occurs, profile abnormalities will result due to areas of incomplete solubility.
Acid loss at the top of the resist profile (the resist/air interface) leads to a bulge in the profile known as a T-top or cap due to reactions with bases adsorbed from the air. Acid loss at the bottom of the resist profile (the resist/substrate interface) leads to profile abnormalities such as resist footing on positive resists, and undercutting on negative resists. The acid loss is believed to be due to the presence of nitrogen-containing compounds such as atmospheric ammonia or ammonium ion which neutralize the photoacid, resulting in incomplete solubility of the resist. It is known that the presence of organic compounds such as amines in concentrations as low as 100 ppb adversely affects the stability of some CA resists.
Methods for reducing the amount of nitrogen-containing compounds on a CA resist substrate have included the use of a barrier layer such as an anti-reflective coating, treatment with oxygen plasma, and increasing the exposure dose. While use of these known methods produces some improvement over an untreated substrate, significant profile defects are still evident.
There is needed, therefore, a method for reducing resist profile defects when a chemically amplified resist such as a DUV resist is formed. In particular, a method for reducing contamination on the substrate before the formation of a CA photoresist layer is needed.
The present invention provides a method for reducing resist profile defects by reducing contamination on a substrate before the formation of a CA photoresist layer. A substrate provided with a first layer to be etched and an anti-reflective coating layer is exposed to a cleansing etchant to remove contaminants prior to the formation of a CA photoresist layer thereon. The cleansing etchant is a solution of sulfuric acid and an oxidizing agent, which may be hydrogen peroxide, ammonium persulfate, potassium persulfate, or the like. The cleansing etchant is known in the chemical arts as xe2x80x9cpiranhaxe2x80x9d or Caro""s acid. After treatment with the cleansing etchant for a predetermined time at an elevated temperature, a CA photoresist layer is then applied to the substrate.
Advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention.